Fighter. In fact, he's so dumb he can't even BE killed.
Black Mage also qualifies, most famously for his fondness of solving every problem with nuclear blasts, having apparently once used it on a bee. It gets worse when it turns out that he can only use his nuke spell once per day, and favors using his knife over other spells (he repeatedly tries to kill Fighter with it, with no success), and when once suggested that he use a lower-than-level-9 spell, he said they weren't his idiom. Aside from this, there's also his attempts at hitting on White Mage, which usually result in her beating him to a pulp. Red Mage also falls under this, once being too dumb to use white magic on himself after getting his ass kicked by werewolvesnote He didn't want to use up the spell, you see., and once not noticing a bridge across lava that Fighter and Black Mage notice. Even Thief gets this as this strip shows. In spite of this, all four of them have some moments of intelligence.
The entire order of the Red Mages, there's a reason Red Mage is The Last of His Kind. You see, they held their elaborate, secret meetings, while other people were busy finding mates and reproducing. This, combined with their trying to find the underlaying rules of how the universe worked by hitting each other with random weapons and spells to see how much damage they did, sort of resulted in, well... as Muffin so aptly put it:
Muffin: You stupided yourselves into extinction.
You can't forget Black Belt, who once got so lost travelling down a straight hallway that he ended up tearing a hole in space/time, resulting in him being followed around by a time duplicate of himself from one second in the past.
Bikke started out merely very stupid, with things like refusing to repair his ship while sailing on a monster-filled sea and feeding his crew nothing but Cheetos, and later evolved to the point where even Fighter appeared more intelligent.
In Antics, Fletcher is often seen doing completely moronic things, such as doing a back-flip onto a glass table, bringing nuclear warheads into his house, or forgetting to put the lid on the radioactive waste.
Kaalinor of anti-HEROES. Fortunately, he's already dead, so his stupidity can't cause him further harm.
Bob and George parodies this by noting that one of the main characters has the "extraordinary ability to not recognize life-threatening injuries." In other words - he's too stupid to die.
With full knowledge that the device before him is a bomb (as it gets hinted that he himself planted it there) and with apparent full knowledge of how to disarm said bomb, Captain Broadbandstill comes to the conclusion that the best way to resolve the situation is to treat the bomb like a PSP and punch it out of anger for the square button not working correctly. He survives, though the next issue reminds us that he had died in the previous issue.
Zeromus in Captain SNES falls for every stupid trick in the book, mostly because he's a personification of protagonist Alex's hatred. It just so happens that that the thing Alex hates most is stupid people.
Casey and Andy may be brilliant inventors, but they're literally Too Dumb to Live, since they get killed constantly. Trick juggling near unprotected anti-matter, skydiving but forgetting to pack the chutes, the wood-powered submarine with the chimney... The list is WAY too long.
Gordon Frohman from the Half-Life-based comic Concerned is quite possibly the definition of Too Dumb to Live as it's revealed near the end of the comic that he's been playing the entire game with the Buddha cheat on this whole time; he then turns it off, with predictable results.
Crusader Amulet. Adventurer tries to recruit a soldier into his Guild. The soldier is apart of the army that Adventurer killed 300 men alone. The soldier refuses and gets killed by Assassin.
Ethan of Ctrl+Alt+Del. According to Zeke, if you run the data from Ethan's various misadventures, his life expectancy comes out negative. Negative forty.
In Darths & Droids, Jim plays Han Solo as a complete moron who brings himself and his team headlong to death's door.
The time-traveling paleontologist in a strip fromDawn of Time. He fails to consider that he might be wrong about the T. rex being a so-called "pure scavenger" — or that even if it were, there hasn't ever been a scavenger on Earth "pure" enough to avoid eating a small, easy-to-kill creature that's not even trying to get away.
The Elves in Errant Story, with exactly two exceptions (Sarine and Misa [literally the youngest elf in the world]). The aftermath of the final battle brings this trope into full focus, when one of the surviving Elves threatens Meji — who had just saved them all from genocide — that the surviving Elves would not rest until either she gave up the power she "stole" or she was dead. Note that Meji is just as powerful as Ian, the half-elf that nearly killed them all minutes earlier. And they know it. Not a single Elf on the scene so much as protests his words (save Sarine, who's too exhausted and disgusted to muster the energy).
The end of the "Red Herring" arc of Exterminatus Now sums up an entire town's cult as this. After generations of absorbing the entire town into the cult, its members chose to reduce themselves into an artificially sweetened, powdered beverage mix, which was then infused into lime cool-ade for its leader to drink and thus become a gestalt psychic entity. He only got a litre and a half down before he fell into a diabetic coma, dipped into the cannibal juice and drowned. In a universe of powerful daemonic and supernatural entities and eldritch forces at work, none of them were involved in this travesty. In the end, as Eastwood said, "a group of nutters chlorinated, sweetened, and artificially flavored their own gene pool".
Background character mentioned only in passing, but: X The Destroyer is definitely an example. He camped on the big plain in front of Mechanicsburg, right on top of the big X.
Also, Snapper, who once he learned "Wilhelm" was actually Othar's sister immediately tried to take her hostage. She then kills him with a single kick and even the other inmates state how stupid this was, earning him a place on this list.
And then there's Dr. Merlot. Okay, not figuring out that Agatha Clay is really Agatha Heterodyne is excusable. But burning down the building with the important papers inside, as well as all the cryptographers that cracked the code they were written in goes a long way beyond the Moral Event Horizon. He's lucky to have only been sentenced to Castle Heterodyne but then he cements his Too Dumb to Live credentials while in there by attacking Agatha and her friends.
The adviser sent by the council to advise Martellus has all of them beaten, though. Tweedle is an unstable bastard at the best of times, but when said adviser, after ordering to fire on the Corbettites as a show of power (which results in the loss of several airships), explains with lots of condescension to Martellus that he's still young and need to be shown how it is done, you know he doesn't have long to live. Tweedle throws him out of the airship before he can even finish his spiel]].
Tempts Fate manages to drown a score of hostile World of Warcraft PCs (It Makes Sense in Context) by telling them that if they submerge into water and wait until they run out of breath, they'll be teleported to a secret place full of loot and XP. And why did they listen to him when only moments before they were after his blood? Because he had an exclamation mark over his head that marked him as a quest-giver, which he stole from an actual quest-giver right before the eyes of said PCs.
Many prey animals in Kevin & Kell display this behavior, such as walking into obvious predator traps, attracting attention to themselves while out in the open. Ray (a firefly) in particular isn't at all bothered by Lindesfarne being an insectivore. Ray is a bit on the dim side. His moth wife Tammy is theoretically smarter, and she has a close friendship with said insectivore.
Liquid in the early stages of The Last Days of FOXHOUND. At one point, he manages this twice in about five strips by a) stepping into the laser grid that will explode the base they're in, and b) shortly thereafter, cracking wise about deranged telekinetic Psycho Mantis's injuries.
In Next Town Over, when wanted criminal John Henry Hunter gets some time with two hookers, the ladies pull guns on him with the intention of turning him in to collect the massive bounty on his head. They apparently either forgot or didn't hear about his magical control over fire, which, among other things, lets him blow up cigars and fired-off guns in the faces of those holding them.
Hunter: I didn't have neither of you girls pegged for stupid. Maybe little mill town whores aren't much for readin'. But I gotta wonder how the wanted poster impressed upon you the face, the name, and the sum while not conveyin'... not to fuck around with me even if I give the impression of vulnerability.
Elan, early on in the story: "Bluff, Bluff, Bluff, Bluff the stupid ogre!" Summed up by the team leader:
Roy: I tend to see Elan more as an obstacle that this team overcomes on a regular basis. Traveling with Elan is kind of like, say, adventuring with syphilis. It can be done, for a while, but it's not easy and it's not pretty.
Some fans also think that Celia the Sylph is this by way of Stupid Good, due to her pacifism and her willingness to do things like ignore the obviously evil surroundings of Greysky City, and Haley's explicit warnings, and wander off on her own with Roy's body. Haley certainly thinks so:
Haley: How can you be so smart sometimes and still be such... an airhead!?
Crystal of the Thieves' Guild is described by Rich in the commentaries as being too dumb to live, and really only survives because she's got Bozzak thinking for her. The comics repeatedly demonstrate this fact.
Tsukiko, necromancer (and necrophile) and one of Xykon's lieutenants, thought that because Redcloak tolerated her constant taunts and attempts to undermine his authority he was a submissive coward. So when she found out that he was betraying their master, she told him that she knew and was going to alert Xykon, expecting him to stand meekly aside and allow it (and if not, just run away because she's got minions to intercept him and teleportation spells to outmaneuver him). What really makes this Too Dumb to Live is that even if Redcloak was submissive, telling him that you're going to get him killed would certainly provoke a response — even a docile animal can be dangerous when cornered. Plus, Redcloak has repeatedly demonstrated a Crazy-Prepared nature, so even those teleportation spells she could have used to evade him could have been (and were) countered.
Nale proves himself to be this when he tells his father and Laurin, who are both far stronger than him, to their faces that he killed Malack, who was an old adventuring friend of theirs and Tarquin's best friend, and that he suffered as he died. Laurin is clearly enraged by this, and when Nale tells his father that he wants nothing from him, including his protection, Tarquin kills him as payback for killing Malack. For good measure, Laurin disintegrates his body so he can't be resurrected.
First with a crew of "space pirates" who manage to get eaten by their third would-be hijacking victim.
Then the blue-skinnedFederation aliens who run their ship with an exposed antimatter reactor, have crackerbox computer security, fly shuttlecraft with the aerodynamics of a cement block, and use matter-transporter technology despite having at least one crew member who has been grotesquely mutated and deformed by its chronic use....
In Book 10, the inhabitants of the Credomar Habitat use fuel-air explosives inside their space station as part of a protest march.
And then there's the ones who kept enough anti-matter around to create an eighty-megaton explosion and didn't even bother to fire-proof the containers. As it turns out, fuel-air explosives and fullerened anti-matter don't mix...
Torg (and sometimes Riff) take this role occasionally in Sluggy Freelance. Probably the most extreme example was when they summoned a demon with the power to destroy the world just so it would give them a case of beer and $20 in cash. If the demon hadn't been a few cents shy of the full twenty, the series would have been a lot shorter.
Apparently, the two kids in thisxkcd comic are just bright enough to recognize Indian bones but not bright enough to recognize an Indian Burial Ground until after they've desecrated it (and no doubt invited all sorts of well-deserved supernatural horrors onto themselves — check out the Alt Text for more fun).
This blogger makes a series of quick comics of Skyrim. About half of those are this trope.